The present invention relates generally to the field of mercury emission control in coal-fired systems, and, in particular, to a new and useful control apparatus, method of operation and potential applications for the apparatus for the removal of mercury from flue gas where more effective removal of elemental mercury is obtained by oxidation with high-energy-dissociated halogens brought into contact with the flue gas, with or without the addition of sorbents such as carbon.
Mercury (Hg) emissions have become a health and environmental concern because of their toxicity and ability to bioaccumulate. The U.S. Environmental Protection Agency (EPA) has recently determined that regulation of Hg emissions from coal-fired electric power plants is necessary and appropriate. Recently enacted clean air regulation seek to phase in more stringent mercury emissions over the next several years, creating an urgent need to develop more effective mercury control technologies.
Mercury in flue gas can be captured by injection of sorbents such as carbon, which are removed by subsequent particulate collection devices. Although sorbent injection is, so far, the most mature control technology, the amount of sorbent needed to serve the U.S. market is expected to be large and economically burdensome to implement and maintain. There is a need to develop new methods to minimize changes required for utilities and to reduce costs associated with capital equipment and carbon injection.
Flue gas constituents, especially halogens or halides, can impact the fate and form of mercury in the flue gas. Naturally occurring chlorine in coals or halogen compounds that have been added to the fuel are converted in the furnace to the atomic form but, being highly reactive, react with flue gas components and each other to form more complex molecular forms. For example, when a halogen such as chlorine is used, reactions with water vapor, SO2, and other flue gas components will occur and will form products such as HCl, SO2Cl2, and Cl2. As the flue gas cools, reactions of atomic or molecular halogens with elemental mercury will also occur, but are limited depending on other competing reactions discussed above. Heterogeneous reactions with or on particulates can occur in addition to gas-phase reactions.
Reactions of atomic halogen species generated in the furnace are kinetically limited and heavily depend on temperature-time profile. The issue is the amount and form of halogen available for oxidation of Hg0 in the gas phase, or for interaction on the surface of a sorbent.
Horne (Horne, D. G.; Gosavi, R.; Strausz, O. P. J. Chem. Phys. 1968, 48, 4758.) determined a rate constant for a Hg+Cl atom by measuring the formation of HgCl using a spectroscopic (279 nm) method for this product. The second-order rate constant for this Hg(I) species was about 1.3×10−11 cm3 molecules−1sec−1. This very fast reaction could be followed by a second reaction to form HgCl2. Using indirect methods, Ariya (Ariya, P. A.; Khalizov, A.; Gidas, A. J. Phys. Chem. A 2002, 106, 7310) determined second-order rate constants for Hg0 with halogen species as follows: with a Cl atom, 1.0×10−11 cm3 molecules−1sec−1; with a Br atom, 3.2×10−12 cm3 molecules−1sec−1; with Br2, 9×10−17 cm3 molecules sec1; and with Cl2, 2.6×10−18 cm3 molecules−1sec−1. Thus the atomic Cl rate constant is about four million times higher than Cl2. However, under a typical temperature profile of a coal-fired utility plant, the atomic Cl generated in the combustion zone has already reacted with other flue gas constituents or itself before it could oxidize elemental mercury at required temperatures.
Because halogen reactivity with mercury is a key factor in control, basic research in this area has been conducted by several investigators. Mamani-Paco and Heble (Mamani-Paco, R. M.; Helble, J. J. In Proceedings of the A&WMA Annual Conf.; Salt Lake City, AWMA: Pittsburgh, 2000) studied the oxidation of Hg with injected HCl and Cl2 using a quenching system comprising a gradient temperature reaction tube from which samples could be withdrawn for analysis. No oxidation occurred using realistic quench rates with 100 ppm HCl. This is expected: HCl is not an oxidizing agent, since it is already in a highly reduced form. Using a composition containing 50 ppm Cl2 gave only 10% oxidation of Hg0, while very large amounts (500 ppm) of Cl2 gave 92% oxidation. The implication is that the more reactive atomic chlorine was not available in the system. Sliger et al. (Sliger, R. N.; Kramlich, J. C.; Marinov, N. M. Fuel Process. Technol. 2000, 65-66, 423) injected HCl and Hg(II) acetate into a natural gas flame and obtained oxidation data consistent with the formation of atomic chlorine and subsequent reaction of a superequilibrium concentration of atomic chlorine with Hg0 at temperatures of 400° to 700° C. Using a very fast (10×) fast quench rate, Niksa reported up to 40% oxidation at 300 ppm Cl (see: Niksa, S.; Fujiwara, N. Prepr. Pap. Am. Chem. Soc., Div. Fuel Chem. 2003, 48 (2), 768).
The Energy & Environmental Research Center (EERC) recently demonstrated significant mercury enrichment in ash when HCl was fed into a high-temperature environment followed by a superfast quenching rate of ˜5400° C./s (Zhuang, Y.; Thompson, J. S.; Zygarlicke, C. J.; Galbreath, K. C.; Pavlish, J. P. in Proceeding of Air Quality IV, Mercury, Trace Elements, and Particulate Matter Conference; Sep. 22-24, 2003). Only 6% of the elemental form was not oxidized and converted to particulate forms. This experiment implies that atomic chlorine generated in the hot zone was still available at lower temperatures, and thus oxidized the mercury at a lower temperature where Hg—Cl reactions are most probable. While in the end most of the mercury was on the ash, it was not clear whether oxidation occurred in the gas phase or solid phase, or where on the solid phase. More recent results from EERC suggest that a significant portion of mercury is oxidized by reactive halogens in a heterogeneous reaction on carbon particulate surfaces. The initial product of the atomic chlorine reaction with Hg is HgCl, which would readily collect on ash, carbon, or sorbent particulates or react with other species or itself.
Further EERC pilot-scale experimental data (Zhuang, Y.; Pavlish, J. H.; Holmes, M. J.; Benson, S. A. Pilot-Scale Study of Advanced Mercury Control Technologies for Lignite-Fired Power Plant in a Retrofit Advanced Hybrid Filter, Proceedings of the 29th International Technical Conference on Coal Utilization & Fuel Systems; 2004, Vol. 2, pp 753-764) showed that the reactive halogen species formed at high temperatures can not only significantly enhance mercury oxidation but also improve the reactivity of mercury with activated carbon. The atomic, radical, and/or molecular halogen species that are formed, at least momentarily, in the high-energy environment react at high rates with mercury both as gas-phase and solid-gas interactions. Flue gas-quenching rates also play a role in the mercury-halogen chemistry. A high flue gas-quenching rate will preserve the reactive halogen species formed in the high-temperature zone for ongoing mercury oxidation and gas-to-particle conversion.
Recent bench, pilot, and full-scale experimental data at the EERC demonstrate that halogen species can vastly improve mercury capture kinetics and overall control performance of sorbents. Halogen species are able to improve the reactivity of the sorbent surface; thereby increasing the sorbents ability to remove mercury from the flue gas stream.
Part of the insight of the present invention is that because of high reaction rates, it is difficult to preserve reactive halogen species generated in the furnace so that these forms will be available for reaction with Hg0 at lower temperatures where the resulting mercury compounds will be stable and more easily captured. The present invention reduces the fundamental teachings of the prior art concerning formation, addition, and rapid transfer of atomic, radical, and/or molecular halogen species to a combustion flue gas or product gas from a gasification system into a practical and effective method for mercury control in a utility flue gas stream.